The present invention relates to a read system for reading information from a magnetic storage medium using a magnetoresistive head and for providing an output signal representative of the information read. In particular, the present invention relates to a read system with improved bandwidth and high frequency noise performance.
A popular method of magnetic data storage utilizes magnetoresistive (MR) heads to store and recover data on a magnetic data storage medium such as a magnetic disk. An MR head employs an MR element that changes in resistivity with changing magnetic flux from data patterns on an adjacent magnetic disk surface. A bias current having a constant value is passed through the MR element, and the change in resistivity is measured by sensing the change in voltage across the MR head.
Amplifier circuits that sense signals from MR heads commonly include differential inputs and differential outputs. While there are a wide variety of differential amplifier circuit topologies, most include an input stage with two load resistors and symmetrical transistors for splitting current between the load resistors. Usually, the output voltage is taken as the difference in the voltage drops across the load resistors; in this manner, large variations in output voltages may be achieved with extremely small input voltage differentials.
For all differential amplifier circuits there are associated therewith certain frequency response performance characteristics. These characteristics and others determine the usefulness of the amplifier circuit in any given application. The band of frequencies over which the gain of the amplifier circuit is almost constant is called the bandwidth. Signals whose frequencies are outside the bandwidth will experience lower gain, with the gain decreasing as the signals move farther away from the bandwidth. Normally, the amplifier circuit is designed so that its bandwidth coincides with the spectrum of signals it is required to amplify. If this were not the case, the amplifier circuit would distort the frequency spectrum of the input signal, with different components of the input signal being amplified by different amounts.
One well-known modification to the differential amplifier circuit is the addition of two capacitors that are cross-coupled to the transistors of the input stage. The capacitive (or ac) cross-coupling causes the noise resistances of the input transistors to be connected in parallel instead of in series, thereby reducing the effective noise resistances of the input transistors. An example of such a circuit is shown in U.S. Pat. No. 5,559,646. However, the main disadvantage of this type of circuit is its limited bandwidth. Because the cross-coupled capacitors cause the input capacitances of the input transistors to be connected in parallel, the effective input capacitances of the input transistors are increased. Therefore, because upper cutoff frequency is inversely proportional to input capacitance for transistors, the high frequency bandwidth of the circuit is reduced and the high frequency noise is increased.
Accordingly, there is a need for a differential amplifier circuit having a cross-coupled input stage with improved bandwidth and high frequency noise performance.
The present invention is a differential amplifier circuit for amplifying an input signal and for providing an output signal representative of the input signal. First and second amplifier circuits each include first and second transistors, a resistor, and a current generator. A first coupling circuit includes a transistor, a capacitor, and a current generator, and couples a first input signal node to the first transistor of the second amplifier circuit. A second coupling circuit includes a transistor, a capacitor, and a current generator, and couples a second input signal node to the first transistor of the first amplifier circuit.